A pressure swing process for obtaining nitrogen is known from DE 29 32 333 C2.
It is characteristic for carbon molecular sieves to preferentially adsorb the oxygen in the air before adsorbing the nitrogen, so that during an adsorption process, nitrogen of high purity can be obtained. The separation takes place through a molecular sieve effect whereby during the adsorption step the smaller oxygen molecules enter the finest pores of the carbon molecular sieve, while the larger nitrogen molecules can barely enter these pores or are excluded therefrom. As soon as an adsorption bed filled with carbon molecular sieves is extensively saturated with oxygen, the carbon molecular sieve is regenerated in the negative pressure range by relieving the pressure, e.g. to 1 bar ambient pressure, or by applying a vacuum. The oxygen molecules are again removed from the pores of the carbon molecular sieve, whereafter a new adsorption step can be started.
The pressure swing process for obtaining nitrogen from air or nitrogen-containing gases is generally makes use of two or more alternately run adsorbers, wherein the steps of adsorption and desorption are alternately carried out.
In order to make the process more economical, pressure compensation can be introduced between the adsorption and desorption steps, as taught by EP 0 121 042 A2, whereby the lower ends and the upper ends of the adsorber are short-circuited by pipelines, in order to enhance the efficiency of the process. Prior to the renewed adsorption step, a part of the produced product nitrogen can be returned in counterflow to the adsorber, in order to obtain a product gas of higher purity (compare for instance DE 34 33 058 A1).
According to EP 0 085 155 A1, the processes for obtaining nitrogen from air or nitrogen-containing gases by means of carbon molecular sieves are generally performed with dry or at least predried air, because under the influence of air humidity the specific nitrogen extraction rate (m.sup.3 N.sub.2 /m.sup.3 CMS h) decreases sharply during the starting period of the pressure swing installation. The abbreviation CMS in the brackets signifies carbon molecular sieve. Even with predried air, e.g. air dried to a pressure dew point (8.5 bar) of +2.degree. C., the specific nitrogen extraction rate decreases according to research made by the applicant by approximately 514 10% in the first weeks of the pressure swing operation. The results are even less favorable when a pressure swing installation filled with carbon molecular sieves is run with 100% saturated air.
In order to keep the negative influence of humidity within limits, cold driers or adsorption driers are used, by means of which the pressure swing installation is dried to technically valid values, e.g. to a pressure dew point (8.5 bar) of +2.degree. C. Such cold or adsorption driers are provided upstream of the pressure swing installation (compare NATO-ASI-Series E: Applied Sciences, Vol. 158 (1989), P. 269/283)
The lower part of the pressure swing adsorber can be filled with a special drying agent, e.g. aluminum oxide or silica gel, in order to keep the humidity as much as possible away from the carbon molecular sieve (compare for instance U.S. Pat. No. 4,203,958). However, one then has to accept the disadvantage that the lower part of the pressure swing adsorber is not available for the separation of the air into oxygen and nitrogen. This leads to a lower purity of the product gas, a reduction of the specific nitrogen extraction rate as well as to an increase of the specific energy consumption, since the dead volume occupied by the drying agent also has to be cyclically brought to adsorption pressure.